As an OCXO (Oven Controlled Xtal Oscillator) that is an oscillator, there is known a configuration that includes two crystal resonators and a heater (a heater circuit) adjacent to these crystal resonators as disclosed in, for example, Patent Document 1. In this OCXO, a temperature of an atmosphere where the crystal resonators are placed is detected by using a frequency difference between the two crystal resonators. Based on the temperature detection result, a supplied electricity to the heater disposed adjacent to the crystal resonators is controlled. Thus, the temperature of the crystal resonators is maintained in a constant range, thereby stabilizing the frequency of the OCXO with respect to the ambient temperature change.
Thus, the OCXO is to achieve a stability of the frequency by maintaining the temperature of the crystal resonator in the constant range. Especially, the OCXO having the above-described configuration performs a further accurate temperature detection by using the frequency difference between the two crystal resonators, thereby achieving a frequency adjustment with high accuracy.
However, when the OCXO having the above-described configuration was actually manufactured, there has been a case where the frequency of the frequency signal that was output from the OCXO was varied exceeding a range of theoretical frequency variation corresponding to a temperature adjustment range of the heater circuit.
Here, in Patent Document 2, there is disclosed an example in which a cap (crystal resonator cover) that airtightly seals a tuning-fork type crystal resonator is constituted of nickel silver (Cu—Ni—Zn based alloy) or phosphor bronze. However, the configuration of the present invention is not disclosed. These materials have a relatively low heat conductivity. Therefore, these materials are not preferred as the material for the crystal resonator cover of the crystal resonator that stabilizes the temperature using the heater circuit.